LONDON —The head of Scotland Yard’s specialist operations department, Andy Hayman, was dining with his family in Spain on Wednesday night when Scotland Yard phoned to suggest that he should return immediately.

He touched down in London shortly before 3 a.m. yesterday to find himself involved in the biggest anti-terrorist operation since last summer’s suicide bombings.

As in those hectic days last July, Mr. Hayman attended meetings of the government’s Cobra Committee, which coordinates its responses to major emergencies and was chaired by John Reid, the home secretary.

Mr. Hayman reported to the committee on the progress of a flurry of arrests around London, the Thames Valley, and Birmingham, which had begun before midnight on Wednesday after his deputy, the head of the anti-terrorist branch, Peter Clarke, and MI5 made a critical decision.

They concluded that a surveillance operation against a network of suspected Islamic terrorists, which began last year, had to be brought to a head.

The threat that they allegedly posed could hardly be overstated: It was a plot to smuggle the components for homemade chemical bombs onto trans-Atlantic jets, possibly 10, in containers such as drink cans or bottles.

The components, thought to be a crystal/powder mix and liquids, would be assembled and set off in midair. In the words of security sources, “apparently benign compounds would be combined to make a malign one.”

The intended explosive material is thought to be a peroxide-based compound similar to that used in the July 7 bombings on the London transport system last year.

Senior officers believe the alleged plotters, most of them British-born and some of Pakistani background, were only a matter of days, possibly two, away from attacks.

The deputy commissioner of the metropolitan police, Paul Stephenson, said: “This was intended to be mass murder on an unimaginable scale.”

Mr. Hayman’s swift return illustrates the urgency that gripped the British security establishment on Wednesday evening. It is understood that a “trigger event,” which has yet to be identified, made police and MI5 realize that the threat to the public was too great to wait any longer.

Speculation is likely to focus on the role of Pakistan, whose intelligence agencies made plain Wednesday that they played a key role in helping Britain. British security sources confirmed this.

Suspects are known to have been arrested in Pakistan.

Whatever the spur, however, the result in Britain was the arrest of 24 suspects, who were booked into Paddington Green high-security station in west London.

The arrests were also accompanied by an extraordinary tightening of security at Britain’s airports.

Department of Transport officials were kept informed of the emerging threat, and security officials at major airports were alerted by the early hours of yesterday that new and stringent checks were required.

The security measures suggest that the threat is still felt to be real, despite all the arrests. Mr. Reid, said at a news conference yesterday: “While the police are confident that the main players have been accounted for, neither they nor the government are in any way complacent.”

Police sources suggested that “all the targets of the arrest operation were arrested.”

Police can hold the suspects for up to 28 days. They will be quizzed in a complex set of interviews aimed at establishing the extent of the suspected plot and its intended targets, as well as obtaining information about associates and sympathizers who may be implicated in the alleged plot.

Inevitably, some people will speculate about “masterminds,” particularly as many of those arrested are thought to be young men, some teenagers and others in their early 20s.

Security sources have pointed to direction, or at least encouragement, of British-based conspirators from Pakistan.

In characteristically understated terms, Deputy Assistant Commissioner Peter Clarke yesterday said the arrests were only the first phase of what may be a lengthy criminal inquiry.

“Today marks the culmination of one phase of what is a major operation that has already lasted several months and will undoubtedly last long into the future,” he said.

Mr. Clarke characterized the investigation as one “focused on intelligence, which suggested that a plot was in existence to blow up trans-Atlantic passenger aircraft, in flight.

“The intelligence suggested that this was to be achieved by means of concealed explosive devices smuggled onto the aircraft in hand baggage.

“The intelligence suggested that the devices were to be constructed in the United Kingdom and taken through British airports. The number, destination, and timing of the flights that might be attacked remain the subject of investigation.”

The police/MI5 operation has emerged as one of the biggest in recent years. Mr. Clarke said: “During the investigation, an unprecedented level of surveillance has been undertaken, and our objective is to gather intelligence and evidence in support of the investigation.

“We have been looking at meetings, movements, travel, spending, and the aspirations of a large group of people. This has involved close cooperation, not only between agencies and police forces in the United Kingdom, but also internationally.

As is so often the case in these investigations, the alleged plot has global dimensions.

“The investigation reached a critical point last night when the decision was made to take urgent action in order to disrupt what we believe was being planned. As always in these cases, the safety of the public was our overriding concern.”

The arrests and raids went on through the night, until 5:00 a.m. Wednesday. On Wednesday night, police were still searching several addresses, including properties in High Wycombe, Walthamstow, east London, and Birmingham.

No reports were issued suggesting that any explosive devices had been found.

American officials suggested the airlines targeted were United, American, and Continental, which fly to New York, Washington, and California.